1. Field of the Invention
This invention relates to a protecting tube for a thermocouple used for measuring a high temperature atmosphere of a gas or a molten metal.
2. Prior Art
The utility of boron nitride at a high temperature atmosphere has hitherto been known and a protecting tube made of boron nitride has been widely used to protect a thermocouple contained therein, since boron nitride withstands an extremely high temperature and is excellent in resistance to thermal shock. Thus, protecting tubes made of boron nitride containing a thermocouple therein have been employed to measure a high temperature gaseous atmosphere of, for example helium or carbon monooxide as well as a molten metal such as copper, silver, magnesium, zinc, aluminium or iron. Boron nitride is advantageous in that it does not react with such molten metal. However, the known boron nitride protecting tube has a disadvantage that boron nitride reacts with platinum, leading to breakdown of the platinum wire when a molded tube of boron nitride is used as the protecting tube for a platinum/platinum-rhodium thermocouple. As a result, the life time of the platinum/platinum-rhodium thermocouple is shortened by the use of a protecting tube made of boron nitride.
Although a thermocouple protecting tube made of alumina having a purity of not less than 99.5% or a composition mainly composed of molybdenum or zirconia has been known, these known materials for protecting tube are not satisfactory in that they are adversely affected by a change in temperature and often to be damaged or broken thereby.